The present invention relates to bolt heaters.
The present invention has a particular application to bolt heaters used in the heating of studs or bolts, such as the bolts of a steam turbine casing. Although the invention is not limited to such an application, the invention will be described in such an environment and use.
It is desirable to heat the bolts used in a steam turbine casing during the fastening and unfastening thereof in that the bolt will elongate due to expansion during heating, allowing the nut to be threaded onto the bolt to a greater degree during the fastening process. When the bolt cools and shrinks, the nut is pulled tighter against the surrounding surface, thus assuring a secure fasting of the nut on the bolt and a clamping of the parts held together by the nut and bolt. In order to ease the removal of the nut from the bolt, such as during the servicing of the turbine, it is helpful to again heat the bolt to elongate it, in order to move the nut away from the surrounding surface, or at least to lessen the force holding the nut against that surface.
It has long been known to heat bolts, for example see U.S. Pat. No. 2,176,601, and to use induction heating to heat such bolts, for example, see U.S. Pat. Nos. 3,771,209 and 5,397,876.
Commonly available resistance type bolt heaters have a junction box, typically a standard hardware type box, to which is attached a handle, a power cable, and a resistance heating element. The resistance type bolt heater presently sold by Power House Tools, Inc., an assignee of the present invention, includes these components, in which the handle has a threaded stud projecting from one end, which is inserted through a non-threaded opening in a removable cover of the junction box, and is held in place on the cover by means of a nut which is secured to the stud from an inside side of the cover. The power cable is attached to an insert which is threaded to the side wall of the junction box, and the resistance heating element is threadingly attached to a bottom of the junction box. Individual wires from the power cable are connected to conductors on the resistance heating element inside the junction box. A power indicator lamp is supplied on the cover to indicate when the resistance heater is being supplied with power.
A standard junction box has several shortcomings which detract from its usefulness as part of a resistance type bolt heater. For example, by requiring the handle to be secured from the inside of the cover adds time required for assembly. The covers have a relatively thin wall thickness which detracts from the stability of the handle/junction box connection, and is insufficiently thick to support a threaded connection of the handle to the cover. The use of a separate insert component for attaching the power cable to the side wall of the junction box adds to assembly time, and becomes a potential point of weakness in the connection of the power cable to the junction box. The attachment of the resistance heater rod to the junction box can be made directly only when a three quarter inch resistance rod is used. When larger resistance rods are used, an adapter piece must be attached to the junction box, and then the rod attached to the adaptor piece, also becoming a potential point of weakness in the connection of the resistance rod to the junction box.
It would be an advance in the art if there were provided an easy to assemble bolt heater which would overcome one or more of the problems and limitations discussed above.